THE DENTAL TISSUES OF THE ORDER RODENTIA. 
55 i 
giilar sigmoidal curve, their general direction being at an angle of 80° with the sur- 
face of the dentine. Immediately on leaving the dentine the layers make a short 
curve upwards and outwards, during which the margins are serrated ; the serrations 
then become indistinct or altogether cease, and the layers pursue a tolerably straight 
course till near their termination, when they turn upwards, after which the component 
fibres become parallel and advance to the sui%.ce at an angle of 33° with the dentine. 
On looking carefully over a longitudinal section, parts will be found in which the 
transverse direction of the lamellse appear to be reversed. Fibres, or layers of fibres, 
will be seen running for a short distance obliquely in the long axis of the tooth. 
In a transverse section the fibres of the alternate layers are seen to cross each other 
obliquely, so as to produce a diamond pattern over the lamelliform portion of the 
enamel. In the earlier part of their course the fibres curve a little, but afterwards 
become straight. In the outer part of the enamel the fibres pass directly outwards 
without leaning to the one side or the other. 
In the lower incisors the structure of the enamel not unfrequently presents a con- 
fused appearance, as though the lamellse were subject to some irregularity in arrange- 
ment. In a longitudinal section the enamel layers are seen leaving the surface of 
the dentine at an angle of 50°, and in their course curve a little outwards. The mar- 
gins are pretty strongly marked with small oblique serrations and oblique transverse 
lines. The lamellse have a thickness of about the 5000th of an inch. The enamel 
has a thickness of about the xwolhs, the lamelliform portion of which is about the 
x /o oths of an inch. In a transverse section parallel with the course of the lamellse, 
the decussation of the alternate layers of fibres may be seen ; and in a section a little 
more oblique, the fibres in the terminal part of their course may be seen passing in 
straight lines obliquely outwards from the median side of the tooth. In the dentine 
of the lower incisors, the vascular canals are much less abundant than in that of the 
corresponding upper teeth. 
The confluent denticles of the molar teeth are coated with enamel, which is very 
irregularly lamelliform, with serrations so indistinctly marked and inconstant, that the 
appearance scarcely merits the name. A longitudinal section will best exhibit this 
view. In a transverse section, parts may be found in which the decussation of ad- 
joining layers of fibres is visible, but generally the fibres seem to follow a much less 
regular and constant course ; and in places they are parallel, as in the outer part of the 
enamel of the incisors. However, when we find the decussation, it will be situated 
on the anterior surface of the denticles ; so far the structure resembles that found 
in the corresponding teeth of the Water-Rat, and, as in that animal, the enamel on 
the posterior surface of the denticles is uniformly free from the lamelliform arrange- 
ment. Near the masticating surface of the molar teeth the enamel fibres are so 
intimately blended, that little or no structure can be discerned. 
The cementum, in no part of the tooth very abundant, is occupied by very large 
irregular cells and vascular canals. The latter are very large, have nodulated mar- 
